1. Field of the Invention
The present invention relates to a method for forming an emitter structure, for example an emitter of a photovoltaic device, and to emitter structures resulting therefrom, as well as to devices comprising such emitter structures.
2. Description of the Related Technology
The emitter of crystalline silicon solar cells is in most cases formed by means of a diffusion process. For example, in case of bulk crystalline silicon solar cells made on p-type silicon wafers, an n-type dopant (such as e.g. P) is diffused into the wafer, wherein the n-type dopant overcompensates the p-type dopant (e.g. B or Ga) of the silicon wafer such that a p-n junction is formed. The emitter diffusion process is usually performed at the front surface of the photovoltaic cell, after texturing of the front surface (the front surface being the face of the photovoltaic cell that is intended for receiving illumination). The main aim of this texturing is to decrease the reflection of light at the front surface. The emitter diffusion process results typically in a Gaussian distribution profile of the n-type dopant, with a high dopant concentration (e.g. ˜1020 cm−3) at the front surface. This high surface concentration is needed to achieve a good contact resistance for the front side metal grid, especially when the metal grid is formed by means of an industrial screen printing process. However, the high surface concentration also results in a poor short-wavelength response due to heavy doping effects and increased recombination in the emitter region.
In order to improve the short-wavelength response, a selective emitter structure can be used (as e.g. described in U.S. Pat. No. 6,552,414), wherein a highly doped emitter region is provided underneath the metal grid and wherein a lightly doped emitter region is provided between the metal contacts of the grid, the lightly doped emitter region resulting in a lower surface recombination and an enhanced blue response as compared to a highly doped emitter. However, selective emitter structures require additional processing steps that may include lithography, which is not industrially applicable for large scale production of solar cells. Moreover, misalignment of the metal grid with respect to the highly doped emitter region of the selective emitter structure can lead to junction leakage resulting from metal penetration through the junction, and to a reduction in open-circuit voltage as a result of this leakage.
As an alternative for forming the emitter of a photovoltaic cell by diffusion, it is also possible to grow the emitter on top of the substrate, for example by chemical vapor deposition (CVD), as for example described for epitaxial cells by Schmich et al. in “Emitter Epitaxy for Crystalline Silicon Thin-Film Solar Cells”, Proceedings of the 21st European Photovoltaic Solar Energy Conference and Exhibition, Dresden, 2006, page 734. The formation of a CVD-grown emitter is realized on a non-textured surface. Therefore, losses by reflection at the front surface are relatively high.